Previous studies of graft-versus-host disease (GVHD) in animal models and humans have been hampered by difficulty in the identification of donor T cell populations according to their ability to recognize recipient alloantigens. This problem has been solved by developing a novel murine model in which an identifiable donor CD8 T cell population recognizes a recipient alloantigen and remains functionally active and pathogenic in vivo for a prolonged period of time after adoptive transfer into lethally irradiated allogeneic recipients. Results of preliminary studies have shown that intermediate avidity stimulation of T cell receptors on donor CD8 cells leads to sustained GVHD, but high avidity stimulation leads to an abortive graft-versus-host response that peaked at day 14 and subsided by day 28 after the transplant. Donor CD8 cells that recognize a recipient alloantigen were present in the spleen and lymph nodes on day 28 after the transplant in recipients with an abortive graft-versus-host response, indicating that activation-induced apoptosis did not account for the resolution of GVHD. Experiments proposed in Specific Aim 1 will test three hypotheses that might explain why donor CD8 cells having a TCR with high avidity for a recipient alloantigen did not cause sustained GVHD. High avidity stimulation of donor CD8 cells might lead to loss of effector function, accelerated elimination of recipient antigen-presenting cells, or altered migration of CD8 effector cells, thereby terminating the progression of GVHD. Experiments proposed in Specific Aim 2 will evaluate the effects of inflammatory mediators on donor CD8 cells that cause GVHD. Previous studies have elucidated the role of T cell-derived inflammatory cytokines in the activation of tissue macrophages leading GVHD, but more recent studies have emphasized the effects of inflammatory cytokines on the survival and function of activated T cells. Experiments have been designed to test the hypothesis that activation in the presence of inflammatory mediators enhances the survival and function of donor CD8 cells that recognize recipient alloantigens, thereby exacerbating the severity of GVHD. Experiments proposed in Specific Aim 3 will test the hypothesis that a CD8 response initiated from memory cells leads to more severe GVHD than a response initiated from naive cells. Experiments proposed in Specific Aim 4 have been designed to test the hypothesis that CD4 help will exacerbate the severity of GVHD by enhancing the survival and function of CD8 cells. The studies proposed in this application will help clarify the critical functions and fate of specific donor CD8 cell population that cause GVHD.